In recent years, the petroleum industry has invested heavily in the development of improved marine survey techniques and seismic data processing methods in order to increase the resolution and accuracy of seismic images of subterranean formations. Marine surveys illuminate a subterranean formation located beneath a body of water with acoustic signals produced by one or more submerged sources. A source may be composed of an array of source elements, such as air guns or marine vibrators. The acoustic signals travel down through the water and into the subterranean formation. At interfaces between different types of rock or sediment of the subterranean formation, a portion of the acoustic signal energy may be refracted, a portion may be transmitted, and a portion may be reflected back toward the formation surface and into the body of water. A typical marine survey is carried out with a survey vessel that passes over the illuminated subterranean formation while towing elongated cable-like structures called streamers. The streamers may be equipped with a number of receivers for detecting and/or measuring seismic energy. Often, the receivers may be collocated pressure and particle motion sensors that detect pressure and particle motion wavefields, respectively, associated with the acoustic signals reflected back into the water from the subterranean formation. The pressure sensors may generate seismic data that represents the pressure wavefield (“pressure data”), and the particle motion sensors may generate seismic data that represents the particle motion, particle velocity, or particle acceleration wavefield (“particle motion data”). Equipment on the survey vessel may receive and record the seismic data generated by the receivers.
In a typical marine survey, seismic data is recorded in separate shot records while the survey vessel is moving. Each shot record is created by activating the source elements of a source at the same time or alternatively within a short activation time interval (e.g., about 1-3 seconds) followed by recording resultant wavefields in a longer recording time interval (e.g., about 8-12 seconds). The seismic data recorded in a shot record is treated in seismic data processing as if the seismic data was created as a result of a single source activation at the beginning of the shot record. In reality, however, the seismic data recorded in a shot record typically includes remnant energy from earlier source activations. This remnant energy is called “shot-generated noise” that appears in the final seismic images. In addition, the total wavefield generated by a source and the portion of the wavefield that propagates upward and is reflected down from the free surface, called the “source ghost,” is limited in spectral content by the source elements, the depth of the source elements, the relative positions of the source elements with respect to each other, and the distance between source activations. In a typical marine seismic survey, a source with the same configuration is activated for every shot record. As a result, the spectral constraints imposed by the source element configuration are present throughout the marine seismic survey. Geophysicists and those working in marine seismology seek methods and systems that overcome the spectral constraints and the shot generated noise imposed by conventional source element configurations and activation.